The present invention relates to a control method for a hydraulic drive which is moved via a control branch, and to a control device corresponding thereto.
In general, a high pressure is applied to a hydraulic drive. Because of the cylinder rod seals, moving the drive therefore necessitates over-coming a high friction force which is always directed against the direction of movement. When the hydraulic drive changes its direction of movement, the force value applied to the hydraulic drive must be changed by twice the friction force. This lasts a certain time. The hydraulic drive is stationary during this time interval.
The control branch has at least one position controller, sometimes with a subordinate speed controller, but sometimes also only a speed controller. The controllers frequently have an integral-action component. Any integral-action component possibly present assumes a relatively large value during the standstill of the hydraulic drive.
Particularly in the case of a position controller, there is also the risk that the hydraulic drive will overshoot when approaching a new desired position. The hydraulic drive must then be moved back in the opposite direction. There is then the risk during approach of renewed overshooting, this time in the opposite direction of movement. It is therefore difficult to approach a desired position accurately. Consequently, for the purpose of avoiding a permanent oscillation of the hydraulic drive about its desired position a dead zone of the integral-action component is frequently introduced in the prior art. Thus, a system deviationxe2x80x94albeit a slight onexe2x80x94is tolerated. A PT1 element used instead of the integral-action component also acts similarly: by virtue of the fact that there must be a desired/actual deviation in order to generate a non-vanishing signal at the controller output, a system deviation is tolerated here, as well.
It is an object of the present invention to provide a control method for a hydraulic drive, and a control device corresponding thereto, by means of which the time interval during which the hydraulic drive is stationary can be reduced. Furthermore, it is to be possible to approach a desired position more accurately in the case of a position controller.
This object is achieved of the control method by virtue of the fact that it is determined whether the hydraulic drive has come to a standstill, and that, on detection of a standstill, a force value dependent on the desired direction of movement is applied to the hydraulic drive via a force precontroller connected in parallel with the control branch as a function of the desired direction of movement.
In a version corresponding to this, the object of the control device is achieved by virtue of the fact that it has a standstill determiner for determining a standstill of the hydraulic drive, that connected in parallel with the control branch is a force precontroller which can be fed an output signal of the standstill determiner and a desired direction of movement, and that upon detection of a standstill a force value dependent on the desired direction of movement can be applied to the hydraulic drive by means of the force precontroller.
It is particularly advantageous to apply the present invention when the control branch is prescribed a desired position and an actual position, the hydraulic drive is moved via the control branch under positional control, and the desired direction of movement is determined with the aid of the actual position and the desired position.
It is possible, for example, to determine a standstill of the hydraulic drive by virtue of the fact that an actual speed is compared with a threshold speed and standstill is detected when the absolute value of the actual speed undershoots the threshold speed.
The position control branch can be deactivated while the force value dependent on the desired direction of movement is being applied to the hydraulic drive. Alternatively, it is possible that the position control branch remains activated while the force value dependent on the desired direction of movement is being applied to the hydraulic drive.
The force value dependent on the desired direction of movement can be applied to the hydraulic drive via a force controller. Alternatively, it is possible that valve slide desired-position pulses dependent on the desired direction of movement are applied to the hydraulic drive via a pulse generator until the force value dependent on the desired direction of movement is reached.
In the case of a controlled force precontroller, it is possible to compensate a position-dependent reaction of the hydraulic drive to being driven by virtue of the fact that the force controller is fed the actual position, and the force controller amplifies a difference between a desired force value and an actual force value as a function of the actual position. In the case of a controlled application of the force value dependent on the desired direction of movement, in a similar way the actual position is fed to the pulse generator, and the pulse generator determines the valve slide desired-position pulses as a function of the actual position.
If the force controller has a force controller integral-action component and the force controller integral-action component is held or reset when the force controller is deactivated, the precontroller response is optimized when the force precontroller is regulated. In the case of controlled prescription of the force value, the precontrol can be optimized by virtue of the fact that the pulse generator simulates the force value via a force model.
The force value dependent on the desired direction of movement is preferably determined from a load force and a friction force.
If there is a risk of overshooting of the hydraulic drive because of the force precontroller, it is nevertheless possible to approach a desired position accurately when the difference between the force value-dependent on the desired direction of movement and the load force is less than the friction force. The force controller is therefore switched off before the friction force is reached. However, because of the control response hydraulic liquid still flows into the hydraulic drive, with the result that the friction force is built up in its entirety.
If the position control branch has a position control branch integral-action component and the control branch integral-action component is held or reset when the force precontroller is activated, there is no renewed risk of overshooting upon reactivation of the position control branch.